Manufacture of electrolytic tinplate



Feb. 18, 1969 A.T.C.-MlCROAMPS/CM P. ROTHSTEIN ET AL 3,428,534

MANUFACTURE OF ELECTROLYTIC TINPLATE Filed Nov. 5, 1965 TEMPERATURES l6A 500F. o 550F.

575F. C) 600F.

l l l l l HOLDING TIME- secs.

INVENTORS PAUL ROTHSTEIN WALTE R B ATZ their ATTORNEY United StatesPatent 8 Claims Int. 'Cl. C23h /52 ABSTRACT OF THE DISCLOSURE Thebrightening temperature of continuously\ flowbrightened electrolytictinplate is lowered when the speed of travel of the strip is reduced andincreased when the speed of travel of the strip is increased so as tomaintain constant the corrosion resistance of the tinplate as indicatedby its ATC values.

This invention relates to the manufacture of electrolytic tinplate. Itis more particularly concerned with a process of manufacturingcontinuously flow-brightened electrolytic tinplate having uniformappearance and flatness and uniform corrosion resistance not influencedby changes in the speed of strip travel.

Tinplate is conventionally produced by continuously surface conditioningsteel strip, electro-tinning it, heating the tinned strip to atemperature sufficient to flow or melt the tin coating, quenching theheated strip in water, and winding the tinned strip so-formed intocoils. The electrolyte may be either acid or alkaline. These processespermit the plating of a relatively thin tin coating which is relativelyuniform in thickness. Because the thickness of tin on conventionaltinplate is a small fraction of an inch, the amount of tin coating ismore conveniently expressed in terms of its weight in pounds for a basebox of tinplate. The term base box is a measure of area or surface andamounts to 31,360 square inches. Large amounts of electrolytic tinplateare made with coating weights on the order of one-quarter to one poundof tin per base box of tinplate.

Such thin tin coatings must be homogeneous and uniformly distributed,and the tinplate is frequently specially processed before or aftertinning to insure that it has adequate resistance to corrosion byvarious food products. In the past, determination of corrosionresistance of tinplate has been a rather tedious process, but it hasrecently been found that the corrosion resistance of tinplate used forfood packs can be determined by a galvanic test known as the alloy-tincouple test. That test consists of stripping the tin from a sample oftinplate down to the tin-iron alloy surface and measuring the currentdensity developed by a galvanic couple comprising a pure tin electrodeand the sample immersed in grapefruit juice containing 100 ppm. ofsoluble stannous tin at a temperature of about 79 F. The current densityafter 20 hours is measured in micro-amperes per square centimeter andthe figures so obtained are referred to as ATC values. Low ATC valuesindicate good corrosion resistance, and high ATC values indicate poorcorrosion resistance. The ATC test is described in the paper TheAlloy-Tin Couple Test-A New Research Tool by G. G. Kamm, A. R. Willey,R. E. Beese, and J. -L. Krickl, published in Corrosion, Volume 17,February 1961, pages 106-112.

Commercial electrolytic tinplate as produced under varying conditionshas ATC values ranging from perhaps 0.50 micro-amperes per squarecentimeter downward. Conventional tinplate usually displays ATC valuesin the neighborhood of 0.15 to 0.25, but it is possible to produce3,428,534 Patented Feb. 18, 1969 tinplate having considerably lower ATCvalues. An ATC value of 0.07 has been arbitrarily selected asrepresenting superior quality tinplate for food packing purposes.

Electrolytically deposited tin is dull, and it is conventional toconvert the dull or matte finished electro-deposited tin into its brightform by heating the tinned strip to a temperature somewhat above themelting point of tin, which is about 450 F., and then quenching it.However, tinplate is sometimes heated to considerably highertemperatures. U.S. Patent 3,174,917 of Mar. 23, 1965, discloses aprocess in which the electro-deposited tin is heated to a temperature inexcess of 925 F. and it states that the tinplate so produced hasconsiderably improved corrosion resistance. When tinplate is raised totemperatures well above the melting point of tin, the speed at which thetin alloys with the iron base is greatly accelerated and the time atwhich tinplate having lightweight tin coatings is held at elevatedtemperature must be very carefully controlled. Furthermore, the coatingtends to discolor unless it is surrounded by a protective atmosphere,and strip of tinplate thickness quenched from elevated temperaturestends to buckle or deform. This is particularly troublesome if theheating is not uniform across width of the strip.

Strip is conventionally electro-tinned at a relatively constant speedwhich depends primarily on the thickness of coating desired. It is quitecommon to electro-tin strip at a speed of 1000 feet per minute orthereabouts when coatings of one-quarter to one pound per base box oftinplate are deposited. When the end of a coil of strip is reached, itis necessary to slow down the strip and, of course, increase its speedagain to its steady value after a new coil is substituted. If thetinplate is being brightened at conventional temperatures not greatly inexcess of the melting point of tin, this reduction in speed, whichoccurs gradually as the trailing end of the coil is processed, does notnoticeably affect the color or flatness of the tinplate, even though thetime during which the strip is held at the brightening temperature isincreased in inverse proportion to its speed of travel. However, if thetinplate is brightened at elevated temperatures, the trailing end of thecoil may be quite badly discolored or distorted because of the increasedtime during which it is subjected to the high temperatures. We have alsofound that when the tinplate is being brightened at relatively lowtemperatures, its corrosion resistance is quite sensitive to the lengthof time it is held at temperature. The corrosion resistance of thetrailing end of such strip as measured by ATC values, may be verysignificantly higher than the corrosion resistance of the strip formingthe body of the coil which has been brightened at a constant relativelyhigh speed of travel. As the trailing or outside end of the coil is theonly convenient place from which samples of the coiled tinplate can betaken for determination of properties, such samples can be misleading.

It is an object of our invention, therefore, to provide a process forproducing relatively high temperature flowbrightened electrolytictinplate in coils which have no greater discoloration and distortion intheir end portions than in their center portions. It is another objectof our invention to provide such a process in which the corrosionresistance of the end portions of the coil is not different from that ofthe center portion. It is still another object of our invention toprovide a process of brightening temperature control for an acceleratingor decelerating continuous electro-tinning line. Other objects of ourinvention will become evident from the description thereof whichfollows.

We have found that the objects above-mentioned are attained by varyingthe temperature at which the strip is heated for brightening over alimited range in correspondence with the change in strip speed over alimited range. Our process is most conveniently described with respectto the slowdown or deceleration of the line as the trailing end of acoil passes through it, but it is also applicable in reverse order tothe speed-up or acceleration of the line when the leading end of a newcoil is being introduced.

An embodiment of our process presently preferred by us is describedhereinafter as used on a commercial halogen process electro-tinning linewith which we are familiar. In respects other than those to bedescribed, the line is conventional. It produces tinplate ofconventional gauge, that is, on the order of .010 inch in thickness, andcoating weights between one-quarter and one pound per base box. Thedesired strip speed for tinplate of this weight is about 1000 feet perminute. The distance between the exit end of the induction brightenerand the surface of the water in the quench tank is about 21 feet. Whenthe line is changed over from one coil to the following coil, line speedmay be reduced to a value below 300 feet per minute, and then increasedagain to the 1000 feet per minute previously mentioned.

We have found that under these conditions, tinplate brightened atrelatively tow temperatures, on the order of 500 R, which may have ATCvalues on the order of 0.16 micro-ampere per square centimeterthroughout the body of the coil, shows ATC values on the order of .07micro-ampere per square inch at its trailing end. This difference,however, becomes less as the temperature of brightening is increased,and practically disappears when the brightening temperature is raised to600 F.

In our improved process of manufacture, therefore, we brighten the tinby heating it to a temperature on the order of 600 F. as long as thestrip is moving at its normal steady state speed, say 1000 feet perminute, but measure the temperature of the molten tin on the surface ofthe strip as it leaves the heating means and by suitable adjustment ofthe heating means decrease this temperature over a relatively narrowrange between two predetermined speed values when the strip speed isreduced at the end of the coil. When a new coil is introduced and stripspeed is being increased, we repeat the above procedure in the reverseorder.

We have found that when the tin is heated to 600 F., neither coatingdiscoloration nor strip distortion appears upon reduction of strip speedimmediately below 1000 feet per minute. The speed can, in fact, drop toabout 800 feet per minute before correction of temperature is required.At this strip speed we adjust the strip heating means so as to effect agradual decrease of tin temperature directly proportional to strip speedover a limited range of temperature between two predetermined values anda limited range of speed between two predetermined values. We have foundthat below a strip speed of about 300 feet per minute changes in thetemperature of the molten tin have no significant effect on thecorrosion resistance of the tinplate. We therefore confine ourtemperature control to the strip speed range between about 800 feet perminute and 300 feet per minute, these two values being predetermined asabove described. The minimum reduction of tin melting temperatureconsistent with tolerable discoloration and distortion of the productis, of course, desirable to maintain independence of the tinplatecorrosion resistance from changes in strip speed. We find that thisminimum is about 40 F., that is, that the brightening temperature needbe reduced from 600 F., its first predetermined temperature, only to 560F., its second predetermined temperature, as the line speed drops from800 feet per minute to 300 feet per minute.

While We have described our process in terms of strip speed, thesignificant factor, of course, is the time during which the strip isheld at the tin melting temperature. This holding time is inverselyproportional to the speed of the strip and directly proportional to thedistance between the tin melting means and the level of the quenchingmedium in the quench tank. In the tinning line abovedescribed, a stripspeed of 800 feet per minute corresponds to a holding time of about 1.6seconds, and a strip speed of 300 feet per minute corresponds to aholding time of about 4.2 seconds.

The attached figure is a plot of ATC values against holding time forfour different tin brightening temperatures, 500 F., 550 F., 575 F., and600 F. It shows that the curves for the various temperatures convergerapidly as holding time is increased, differing insignificantly at aholding time of 4.2 seconds. It also shows that at the highestbrightening temperature-600 F.the holding time has no significant effecton the ATC values of the tinplate, as has been mentioned, Likewise, thefigure makes evident that tinplate heated to 600 F. for 1.6 seconds hasabout the same corrosion resistance as tinplate heated to between 550 F.and 575 F. for 4.2 seconds.

The temperature of the tin on the strip leaving the melting zone isconveniently determined 'by an optical pyrometer. We have found that thetwo-color optical pyrometer, so-called, is most suitable for temperaturemeasurements in this range. As the range of temperature change is smallwith respect to the temperature Value, the pyrometer must be capable ofprecise temperature determination.

We claim:

1. In the process of manufacturing flow-brightened electrolytic tinplatein which steel strip is continuously moved successively through anelectro-tinning zone, a tin melting zone, and a quenching zone spacedfrom the melting zone, at a speed which periodically is reduced from arelatively constant value to a lower value and then raised to arelatively constant value, whereby the time during which the strip isheld at tin melting temperature is varied in inverse proportion to thespeed of the strip, the improvement comprising measuring the temperatureof the tin coating as it leaves the tin melting Zone, adjusting thattemperature to a first predetermined value, measuring the speed of striptravel, reducing that temperature when the speed of strip travel fallsbelow a first predetermined value and continuing this reduction to asecond predetermined value at a rate directly proportional to the speedof strip travel until that speed reaches a second predetermined l-owervalue.

2. The process of claim 1 in which the first predetermined speed is notless than that which at the first predetermined temperature of the tincoating causes discoloration of the tin.

3. The process of claim 1 in which the second predetermined speed is notmore than that which at the second predetermined temperature of the tincoating produces brightened tinplate having corrosion resistance equalto that of tinplate brightened at the first predetermined speed andfirst predetermined temperature of the tin coating.

4. The process of claim 1 in which the first predetermined speed is notless than that corresponding to a time of about 1.6 seconds.

5. The process of claim 1 in which the second predetermined speed is notmore than that corresponding to a time of about 4.2 seconds.

6. The process of claim 1 in which the first predetermined temperatureis about 600 F.

7. The process of claim 1 in which the second predetermined temperatureis about 560 F.

8. The process of claim 1 including the steps of increasing thetemperature of the tin coating when the speed of strip travel exceedsthe second predetermined value and continuing this increase to the firstpredetermined temperature at a rate directly proportional to the speedof the strip travel until that speed reaches the first predeterminedvalue.

(References on following page) 5 References Cited UNITED STATES PATENTS2,409,431 10/ 1946 Hess 20437.5 2,566,468 9/1951 Taylerson 204-362,576,902 11/1951 Duby 204-36 6 3,285,838 11/1966 Morgan et a1. 20437.53,334,030 8/1967 Notman 20437.5

JOHN H. MACK, Primary Examiner.

T. TUFARIELLO, Assistant Examiner.

U.S. C1. X.R. 148128

